Fixed race condition in 'repo sync -jN' that would open multiple masters.

This fixes the SSH Control Masters to be managed in a thread-safe
fashion.  This is important because "repo sync -jN" uses threads to
sync more than one repository at the same time.  The problem didn't
show up earlier because it was masked if all of the threads tried to
connect to the same host that was used on the "repo init" line.

git_config.py

@@ -18,6 +18,10 @@ import os
 import re
 import subprocess
 import sys
+try:
+  import threading as _threading
+except ImportError:
+  import dummy_threading as _threading
 import time
 from signal import SIGTERM
 from urllib2 import urlopen, HTTPError
@@ -361,10 +365,27 @@ class RefSpec(object):
 _master_processes = []
 _master_keys = set()
 _ssh_master = True
+_master_keys_lock = None
+
+def init_ssh():
+  """Should be called once at the start of repo to init ssh master handling.
+
+  At the moment, all we do is to create our lock.
+  """
+  global _master_keys_lock
+  assert _master_keys_lock is None, "Should only call init_ssh once"
+  _master_keys_lock = _threading.Lock()
 
 def _open_ssh(host, port=None):
   global _ssh_master
 
+  # Acquire the lock.  This is needed to prevent opening multiple masters for
+  # the same host when we're running "repo sync -jN" (for N > 1) _and_ the
+  # manifest <remote fetch="ssh://xyz"> specifies a different host from the
+  # one that was passed to repo init.
+  _master_keys_lock.acquire()
+  try:
+
     # Check to see whether we already think that the master is running; if we
     # think it's already running, return right away.
     if port is not None:
@@ -429,8 +450,12 @@ def _open_ssh(host, port=None):
     _master_keys.add(key)
     time.sleep(1)
     return True
+  finally:
+    _master_keys_lock.release()
 
 def close_ssh():
+  global _master_keys_lock
+
   terminate_ssh_clients()
 
   for p in _master_processes:
@@ -449,6 +474,9 @@ def close_ssh():
     except OSError:
       pass
 
+  # We're done with the lock, so we can delete it.
+  _master_keys_lock = None
+
 URI_SCP = re.compile(r'^([^@:]*@?[^:/]{1,}):')
 URI_ALL = re.compile(r'^([a-z][a-z+]*)://([^@/]*@?[^/]*)/')
 

main.py

@@ -28,7 +28,7 @@ import re
 import sys
 
 from trace import SetTrace
-from git_config import close_ssh
+from git_config import init_ssh, close_ssh
 from command import InteractiveCommand
 from command import MirrorSafeCommand
 from command import PagedCommand
@@ -214,6 +214,7 @@ def _Main(argv):
   repo = _Repo(opt.repodir)
   try:
     try:
+      init_ssh()
       repo._Run(argv)
     finally:
       close_ssh()